Heat oven to 350°.
Melt butter in a medium skillet over medium heat; add onion and thyme. Cook 15 to 20 minutes, stirring occasionally, until browned. Discard thyme sprigs.
In a large bowl, combine cream cheese, sour cream, milk, Worcestershire sauce, garlic powder and sautéed onion. Reserve 3 tbsp of the scallions; stir in remainder. Gently fold in crab and turn out into an 11 x 7-inch baking dish. Bake at 350° for 30 minutes until bubbly around edges.
Garnish with reserved scallions; serve with crackers and celery sticks.
